1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same. In particular, the invention relates to a semiconductor device having plural power supply systems and a method of manufacturing the same.
2. Description of Related Art
As a delivery inspection of an LSI, an IDD test or a logical operational test is executed. In the IDD test, for example, a current that flows between a power supply and a ground is measured through a semiconductor device while all nodes of the semiconductor device are set to a high voltage level. Thus, it is possible to determine whether or not the semiconductor device is defective. For example, if a leak current flows through a transistor, a current amount exceeds a predetermined value, so the transistor is determined to be defective.
Incidentally, a general semiconductor device includes an input/output circuit as an interface circuit, and an inner circuit that receives/transmits signals from/to the input/output circuit. The input/output circuit includes an input buffer connected with an input terminal and an output buffer connected with an output terminal. For example, as shown in FIG. 5, an input buffer circuit 23 uses a pull-down resistor 25 for keeping an input terminal 22 at a low potential. When an IDD of the semiconductor device having the input buffer circuit 23 provided with the pull-down resistor 25 is measured, a current flows from the input terminal 22 to a ground through the pull-down resistor 25, so a measurement value becomes large. In the case of providing the input buffer circuit with a pull-up resistor as well, a measurement value of the IDD test becomes large.
Japanese Unexamined Patent Application Publication No. 2001-133511 discloses a method of measuring an IDD of the semiconductor device having the input buffer circuit provided with the pull-up resistor or pull-down resistor. In the semiconductor device, an operational test control signal common to the input buffer circuit and the output buffer circuit, which have the pull-up resistor or pull-down resistor, is supplied. The operational test control signal is input to a switch connected with the pull-up resistor or pull-down resistor. The operational test control signal is set to a high level (H) during the operational test, and set to a low level (L) during a normal operation. Thus, during the operational test, the pull-up resistor or the pull-down resistor can be separated from a buffer circuit. As described above, pull-up/pull-down control can be executed by inputting an operational test control signal.
Further, Japanese Unexamined Patent Application Publication No. 7-181230 also discloses a method of separating a pull-up or pull-down transistor from an input circuit in a semiconductor device having a multi-power supply system. This publication describes the structure for pull-up/pull-down control of an input/output buffer circuit in an LSI having different power supply systems in the semiconductor device. In the semiconductor device, a buffer circuit provided to one power supply system (referred to as “power supply system A”) is controlled by using the other power supply system (referred to as “power supply system B”).
Incidentally, in recent years, an increase in standby power consumption due to an off-leak current of a transistor has emerged as a problem. This problem is serious in the field of not only battery-driven mobile device but also digital appliance. In order to save the standby power consumption, for example, plural power supply systems are provided inside the LSI. Thus, a power supply is divided into the plural power supply systems, and a power supply system in a non-operating area is turned off to save the power consumption.
However, in the case of turning off one power supply system, the power supply system B, in the above semiconductor device, there arises a problem in that a buffer circuit of the other system, the power supply system A, is out of control. That is, if the power supply system B is turned off, a control signal cannot be input to the buffer circuit of the power supply system A. Accordingly, if the power supply system for controlling the buffer circuit is turned off in order to save the power consumption during the normal operation, the other power supply system is out of control. As discussed above, in the conventional semiconductor device having plural power supply systems, if one power supply system is turned off, the buffer circuit is out of control.